Project Summary Mitochondria and sarcoplasmic reticulum (SR) form close interfaces in cardiomyocytes where the propagation of Ca signals from SR to mitochondria contributes to heart function under physiologic and pathologic conditions. TRIC is a novel class of trimeric intracellular cation channels located at the SR or endoplasmic reticulum (ER) of multiple cell types, which function as counter-ion channels that allow flow of K ions into the SR/ER during the acute phase of Ca release. Genetic ablations or mutations of TRIC channels are associated with hypertension, heart disease, respiratory defects and brittle bone disease. The recent crystal structure of TRIC proteins confirms the homotrimeric architecture of a cation channel. Within the human and mouse genomes, there are two TRIC isoforms, TRIC-A and TRIC-B, that display differential functions in regulation of Ca signaling in excitable and non-excitable cells. While our past research efforts primarily focused on understanding the physiological function for TRIC-A in skeletal and smooth muscle and to a lesser extent the role of TRIC-B in epithelial cells, the role of TRIC in cardiac physiology and disease remains largely unexplored. Here we present evidence that TRIC-A, in addition to modulation of the SR permeability to K ions, functions as an integral component of the physiological control mechanism of Ca signaling in cardiac muscle. We found that the carboxyl-tail domain of TRIC-A interacts with the RyR2 channel to directly modulate Ca release from the SR. The TRIC-A-/- mice develop arrhythmia, severe cardiac hypertrophy and fibrosis following isoproterenol treatment or transverse aortic constriction (TAC) surgery. Isolated TRIC-A-/- cardiomtocytes display mitochondria dysfunction that likely reflects SR Ca overload-induced mitochondria toxicity under stress conditions. As uncontrolled Ca release has been implicated in mitochondrial dysfunction in cardiac pathology, we hypothesize that ?TRIC interaction with RyR2 modulates SR Ca release and crosstalk with mitochondria. Absence of TRIC leads to Ca overload inside SR and causes stress-induced Ca toxicity to mitochondria. The dysregulated SR-mitochondria crosstalk contributes to the development of heart failure?. We further propose that targeting TRIC- mediated SR-mitochondria crosstalk represents a novel means for treatment of cardiovascular diseases. Our experiments designed in this project contain two specific aims: Aim 1 - to define the functional interaction between TRIC-A/B and RyR2 in regulating the cross-talk of Ca signaling from SR to mitochondria in cardiomyocytes under physiologic and pathologic conditions; and Aim 2 - to elucidate the in vivo role of TRIC-A in mediating stress-induced changes in heart function.